An ionization using a plasma of corona discharge, glow discharge or the like is widely used in mass spectrometry in various fields. An explanation will be given as follows of how the ionization by the plasma has been carried out until this time.
Patent Literature 1 describes a method of carrying out an ionization by an atmospheric pressure chemical ionization method. According to the atmospheric pressure chemical ionization method, the ionization is carried out by corona discharge under an atmospheric pressure. A sample molecule is ionized by an ion molecular reaction with a primary ion which is generated by the corona discharge.
Patent Literature 2 describes a method of carrying out an ionization by glow discharge by introducing a sample into a vacuum chamber. A pressure in the vacuum chamber for carrying out the ionization is about 0.1 through 1 Torr (1×10 through 1×102 Pa).
Patent Literature 3 describes a method of carrying out an ionization by a dielectric barrier discharge under an atmospheric pressure. In the dielectric barrier discharge, a low temperature plasma is generated by preventing an elevation of a temperature of a neutral gas or an ion in the plasma by interposing a dielectric between electrodes. According to Patent Literature 3, the plasma which is ejected from a probe under the atmospheric pressure is directly impinged on a sample to thereby carry out the ionization, and a generated ion is introduced to a mass spectrometer.
Nonpatent Literature 1 describes a method of carrying out an ionization by a dielectric barrier discharge which uses helium as a discharge gas in vacuum. According to the method described in Nonpatent Literature 1, a sample and helium are introduced into a vacuum chamber of a mass spectrometer by passing a capillary of a dielectric, and the sample is ionized by the dielectric barrier discharge at a vicinity of an outlet of the capillary on a side of the vacuum chamber. A flow rate of the gas which is introduced into the vacuum chamber is 25 ml/min, and a pumping speed of a pump which extracts the gas from the vacuum chamber is 300 L/s. Therefore, the vacuum chamber and the vicinity of the outlet of the capillary on the side of the vacuum chamber where the ionization occurs are maintained under a high degree of vacuum. There is strongly observed an atom ion which is produced by dissociating a molecule ion under a condition of Nonpatent Literature 1.